Heating of a storage trap

ABSTRACT

A lean NOx trap in an exhaust system of a spark ignition internal combustion engine having a three-way catalyst upstream of the lean NOx trap, and the lean NOx trap connected in series along the exhaust system is heated by modulating the air to fuel ratio (AFR) of the engine cyclically at various frequencies such as 0.5 Hz, 1 Hz, 1.5 Hz, and 2 Hz, so that different portions of the lean NOx trap are heated at different times so as to completely remove sulfur from the lean NOx trap.

FIELD OF THE INVENTION

The present invention relates to the heating of a gas storage trap andin particular of a trap, mounted in the exhaust system of an internalcombustion engine, for storing oxides of nitrogen.

BACKGROUND OF THE INVENTION

When an internal combustion engine is operating in a lean burn mode,there is always an excess of oxygen present in the exhaust system. As aresult, oxides of nitrogen (NOx) present in the exhaust gases while theengine is operating in lean burn mode cannot be reduced by a three-waycatalytic converter. To avoid such NOx gases from being discharged intothe ambient atmosphere, it has been proposed to store them temporarilyin a trap and to release them into the exhaust stream at a later stagewhen the engine is operated with a rich or at least stoichiometricmixture.

NOx traps can be formed as modified three-way catalytic converters inwhich the matrix has additional NOx storage sites consisting of basemetal oxides. In the presence of surplus oxygen and NOx gases, the basemetal oxides are converted to nitrates thereby trapping the NOx gases.In a reducing atmosphere on the other hand, the metal nitrates decomposereleasing the NOx back into the exhaust stream but at this time thefunctioning three-way catalyst will reduce the NOx gases to nitrogenthat can safely be discharged into the atmosphere.

A problem encountered with such traps is that the base metal oxides willalso react with oxides of sulphur to produce sulphates which are morestable compounds than nitrates and do not normally decompose when theengine is again run in a rich or stoichiometric mode. As a result,unless further steps are taken, the trap is poisoned by sulphur which,at least in Europe, is usually present in the fuel.

It has been found that the damage done to a trap by sulphur in the fuelis reversible in the presence of a reducing atmosphere if the trap isheated to above 650° C.

OBJECT OF THE INVENTION

The present invention seeks to provide a method of heating a lean NOxtrap to a sufficiently high temperature to enable it to be desuplhated.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method ofdesulphating a lean NOx trap (LNT) in an exhaust system of a sparkignition internal combustion engine having a three-way catalyst and thelean NOx trap connected in series along the exhaust system, the NOx trapcontaining both NOx and oxygen storage components, the method comprisingmodulating the air to fuel ratio (AFR) calibration of the enginecyclically, first with a lean calibration for a sufficient time toresult in the presence of excess air in the exhaust gases to provideoxygen to be stored within the lean NOx trap, then with a richcalibration for a sufficient time to result in the presence of excessfuel in the exhaust gases initially to react completely with any oxygenstored in the three-way catalyst itself and subsequently for theremainder of the excess fuel to break through the three-way catalyst toreact with the oxygen stored in the NOx trap so as to heat the NOx trapto a temperature in excess of 650° C.

Three-way catalysts currently in use all have oxygen storage capacity tocope with rich AFR excursions. Purely for the purpose of heating the NOxtrap, the three-way catalyst is not required to store oxygen. However,as oxygen storage capacity is invariably present in the three-waycatalyst, its presence must be allowed for in the fuel computation as itis essential for the excess fuel (i.e. unburnt hydrocarbons and carbonmonoxide) to break through the catalytic converter and find its way tothe storage trap if the latter is to be heated.

The concept of alternating between lean and rich calibrations, that isto say modulation of the AFR, is known for example from EP-A-0 627 548,and is used to maintain a mean stoichiometric mixture, as is required bya three-way catalyst. The invention differs from such engine operationin that it requires a much greater degree of AFR modulation and for amore prolonged modulation cycle period because heating the NOx trap to atemperature at which desulphating can occur requires many AFR modulationcycles during each of which oxygen must first be stored in the NOx trapand subsequently fuel break through must occur past the catalyticconverter and reach the NOx trap to react with the stored oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of an engine to which the presentinvention may be applied, and

FIG. 2 is a graph showing the effect on the temperature gradient in thelean NOx trap of altering the frequency of AFR modulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a spark ignition engine 10 having an intake manifold 12 andan exhaust pipe 14. The intake air is regulated by an intake throttle16, which is preferably electronically controlled. Fuel is introducedinto the intake manifold 12 by a fuel metering system 18 and an ignitionsystem 20 supplies spark energy to the engine spark plugs. The intakethrottle 16, the fuel metering system 18 and the ignition system 20 areall controlled by a computerised engine management system 22.

In the exhaust pipe 14, there is arranged a catalytic converter 24containing a three-way catalyst. A NOx trap 26, which is required tooperate at a lower temperature than the catalytic converter 24, isarranged further downstream in the exhaust pipe 14 and is represented asconsisting of three different slices LNT 1 to LNT 3 arranged in serieswith one another in the direction of gas flow. In practice, the threeslices are all part of the same brick but they are considered separatelyherein because the reaction conditions vary along the length of the leanNOx trap.

The engine can be operated in a lean burn mode for fuel economy, duringwhich NOx gases are stored in the NOx trap. The engine can also beoperated in a stoichiometric mode for optimum operation of the three-waycatalyst and in a rich mode for purging the NOx trap of stored NOxgases. Additionally, according to the invention, the engine can beoperated in a modulated AFR mode to heat the NOx trap in order to enableit to be regenerated after exposure to sulphur.

In lean burn operation, the engine management system 22 sets a fuelquantity less than that required for stoichiometry. The three-waycatalyst 24 acts as an oxidation catalyst and uses some of the oxygen toreact with fuel not burned in the engine. Of the remainder of theoxygen, some is stored in the three way catalyst and the excess breaksthrough and flows to the lean NOx trap 26, where it is stored.

The NOx stored in the NOx trap is purged periodically by allowing a fuelspike to break through the catalytic converter 24. This will suffice toregenerate the NOx trap, provided that it has not been poisoned bysulphur in the fuel.

In stoichiometric operation, to ensure proper functioning of the threeway catalyst, the AFR is intentionally modulated on either side ofstoichiometry at a frequency of about 2 Hz, the amplitude of themodulation being typically 0.01 lambda (lambda being the relative excessair ratio). This heats to a small extent the three-way catalyst becauseduring the lean part of the modulation cycle oxygen is stored in thethree-way catalyst and during the rich part of the modulation cycle thestored oxygen reacts with surplus fuel to generate a small amount ofheat within the catalyst. Throughout this mode of operation, fuel doesnot break through the catalytic converter to reach the NOx trap 26.There is no need under these conditions to send fuel spikes to the NOxtrap as NOx gases are already reduced within the three-way catalyst andare not passed on for storage in the trap 26.

In rich operation, excess fuel will reach both the catalytic converter24 and the NOx trap 26. This will purge the NOx trap 26 if it has notbeen poisoned but will not raise the temperature of the NOx trap becausethere will be no oxygen in the exhaust gases at that time and the finitequantity of oxygen stored in the trap will be exhausted and generatelittle heat to have any appreciable effect on the temperature of the NOxtrap.

To heat the NOx trap 26 to enable it to be regenerated after it hassuffered sulphur poisoning requires the trap temperature to be raisedsignificantly above 650° C. The amplitude of modulation of the AFRneeded for this is of the order 0.1 lambda, that is some ten times theamplitude of the AFR modulation for operating the three way catalyst.The frequency of modulation is also important for reasons that willbecome apparent from a study of FIG. 2 showing an example of an AFRmodulated exhaust flow through a lean NOx trap.

If the frequency of AFR modulation is very low, say 0.5 Hz (once everytwo seconds) then the fuel that breaks the three way catalyst willpermeate the entire length of the lean NOx trap and use up all theoxygen that is stored through the three notional slices LNT 1 to LNT 3.All three slices are heated equally by the reaction of the fuel andstored oxygen but the cooling effect of the exhaust gases is felt mostat the front of the NOx trap. Hence a temperature gradient is set up inthe NOx trap and only the slice LNT 3 at the back of the trap reached atemperature in excess of 650° C. sufficient to burn off any sulphur inthe trap. By contrast, if the modulation frequency is high, i.e. 2 Hz ormore, then only the front slice LNT 1 sees the fuel excursions and it isheated up by using up the entire fuel. This results in direct heating ofthe front slice LNT 1 and the heating of the remaining slices LNT 2 andLNT 3 takes place only by contact with the gases heated up in the frontslice. The temperature is not uniformly high in the NOx trap becauseheat is lost to the ambient from the gases as they progress down thetrap 26 with the result slices LNT2 and LNT 3 do not reach asufficiently high temperature to burn off any sulphur. In theintermediate frequency of 1 Hz a high temperature is reached in thesecond and third slices LNT 2 and LNT 3 of the trap but LNT 1 remainstoo cold to burn off any stored sulphur. At 1.5 Hz, the temperaturedistribution is uniform but unfortunately none of the slices is hotenough to burn off sulphur.

To achieve efficient burning off of sulphur from all parts of the leanNOx trap, in the preferred embodiment of the invention, the AFRmodulation is performed at a swept frequency so that the differentslices are heated sequentially and it is not attempted to run at asingle compromise frequency that will result in uniform heating of thetrap 26. Thus, the frequency of AFR modulation may for example be setfirst to a value of say 2 Hz and kept there for a sufficient time topurge the front end of the trap of sulphur. Thereafter, the frequency ischanged to say 1 Hz and kept there until the centre slice LNT 2 ispurged of sulphur. Finally, the frequency is set to 0.5 Hz to clean thelast slice LNT 3 of the NOx trap. It will be appreciated that thesefrequency values will vary from engine to engine and will depend onoperating conditions and are given here only as an illustration.

This mode of operation must be maintained for approximately 5 minutesfor all parts of the NOx trap 26 to reach the desired sulphur purgetemperature.

During the above AFR modulation period, the same steps of storing oxygenthen reacting it with excess fuel will occur in the three-way catalystand it too will be heated. It is therefore desirable to reduce theoxygen storage capacity of the three-way catalyst 24 to avoid it beingheated excessively during this mode of operation. However, as someoxygen storage capacity is needed for other modes of operation, suchheat as will inevitably be produced within the three-way catalyst duringthis mode of operation is not totally wasted as it will heat the gasesreaching the NOx trap 26 and will indirectly heat the trap 26 despitesome heat losses along the exhaust pipe connecting the three-waycatalyst 24 to the NOx trap 26.

Even in the sulphur purge mode, the average value of the AFR should bestoichiometric to avoid undesired engine emissions.

Because changes in AFR affect output torque, to avoid the drivability ofthe vehicle being affected it is possible to modify the spark timing atthe same time as changing the AFR so as to maintain substantiallyconstant torque. If the engine is fitted with an electronic throttle,its position can also be modified in synchronism with changes in the AFRto reduce the effect on engine output torque.

What is claimed is:
 1. A method of desulphating a lean NOx trap (LNT) inan exhaust system of a spark ignition internal combustion engine havinga three-way catalyst and the lean NOx trap connected in series along theexhaust system, the NOx trap containing both NOx and oxygen storagecomponents, the method comprising modulating the air to fuel ratio (AFR)calibration of the engine cyclically, first with a lean calibration fora sufficient time to result in the presence of excess air in the exhaustgases to provide oxygen to be stored within the lean NOx trap, then witha rich calibration for a sufficient time to result in the presence ofexcess fuel in the exhaust gases initially to react completely with anyoxygen stored in the three-way catalyst itself and subsequently to reactwith the oxygen stored in the NOx trap so as to heat the NOx trap to atemperature in excess of 650° C., wherein the AFR is performed at aplurality of frequencies so as to heat different parts of the lean NOxtrap at different times.
 2. A method as claimed in claim 1, wherein thequantity of oxygen reaching the exhaust system during said leancalibration is sufficient to saturate the oxygen storage capacity ofboth the three-way catalyst and the NOx trap but not sufficient to lowerthe temperature of the NOx trap.
 3. A method as claimed in claim 1,wherein the quantity of fuel reaching the exhaust system during saidrich calibration is sufficient to cause break through of fuel past thethree-way catalyst but not sufficient to cause break through of fuelpast the NOx trap.
 4. A method as claimed in claim 1, wherein the engineis operated with an average air to fuel ratio over the lean/rich AFRmodulation cycle that is stoichiometric or leaner than stoichiometric.5. A method as claimed in claim 1, wherein during the lean/rich AFRmodulation cycle to heat the NOx trap, the spark timing and/or theengine throttle position are/is adjusted to avoid changes in engineoutput torque.